1999 to 2001 4.0 heads with casting number 0331 stamped on top of the head between number 3 cylinder and throttle body are prone to cracking.check your coolant bottle to see if it is draining into the enging oil,oil will be milky.this wipes out the cam bearings.there are also sticking exhaust valve issues with several year 4.0's that may be resolved by spraying mopar combustion chamber cleaner (part # 04318001ab)into the throttle body with the engine running. Follow the directions and don't spray it in too fast oR you can cause a hydro lock where a cylinder gets full of the cleaner and bends a connecting rod.change the oil right after starting it back up.my friend spun a bearing in his toyota 4 x 4 after using this. He only drove 3 miles without changing his oil.chrysler mechanics swear by this stuff.it did a great job on my 89 camery and 89 john deer. Didn't help my jeep a bit. Went to a u-pull-it junk yard and got a 98 head which are supposed to flow the best.it's at the machine shop now getting a valve job.2002 AND NEWER 0331 HEADS WERE CAST IN BRAZIL AND ARE SUPPOSED TO BETTER.GOOD LUCK.Frenchhillbill.